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SEMP8965 Datasheet(PDF) 11 Page - SeCoS Halbleitertechnologie GmbH |
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SEMP8965 Datasheet(HTML) 11 Page - SeCoS Halbleitertechnologie GmbH |
11 / 11 page Elektronische Bauelemente SEMP8965 Fast Ultra High-PSRR, Low Noise, Low-Dropout, 600 mA Micropower CMOS Linear Regulator 01-June-2002 Rev. A Page 11 of 11 http://www.SeCoSGmbH.com/ Any changes of specification will not be informed individually. POWER DISSIPATION AND THERMAL SHUTDOWN Thermal overload results from excessive power dissipation that causes the IC junction temperature to increase beyond a safe operating level. The SEMP8965 relies on dedicated thermal shutdown circuitry to limit its total power dissipation. An IC junction temperature TJ exceeding 165°C will trigger the thermal shutdown logic, turning off the P-channel MOS pass transistor. The pass transistor turns on again after the junction cools off by about 30°C. When continuous thermal overload conditions persist, this thermal shutdown action then results in a pulsed waveform at the output of the regulator. The concept of thermal resistance θJA (°C/W) is often used to describe an IC junction’s relative readiness in allowing its thermal energy to dissipate to its ambient air. An IC junction with a low thermal resistance is preferred because it is relatively effective in dissipating its thermal energy to its ambient, thus resulting in a relatively low and desirable junction temperature. The relationship between θJA and TJ is as follows: TJ =θJA (PD) + TA TA is the ambient temperature, and PD is the power generated by the IC and can be written as: PD = IOUT (VIN - VOUT) As the above equations show, it is desirable to work with ICs whose θJA values are small such that TJ does not increase strongly with PD. To avoid thermally overloading the SEMP8965, refrain from exceeding the absolute maximum junction temperature rating of 150°C under continuous operating conditions. Overstressing the regulator with high loading currents and elevated input-to-output differential voltages can increase the IC die temperature significantly. FAULT DETECTION In the event of the occurrence of various fault conditions that cause failure in the output voltage regulation, such as during thermal overload or current limit, the FAULT pin of the SEMP8965 becomes low. Because the FAULT pin connects to the open-drain output of a N-channel MOS transistor, a large pull-up resistor (100kΩ typical) is required to provide the necessary output voltage and yet without compromising the overall power consumption performance of the regulator. The FAULT pin also goes low when the input-to-output differential voltage becomes too small to sustain good load and line regulation at the output. This occurs typically during near dropout when the input-to-output differential voltage is less than 110mV for a load current of 200mA. The SEMP8965 detects near dropout conditions by comparing the differential voltage against a predefined differential threshold that is always slightly above the dropout voltage. This differential threshold is dynamical in the sense that it not only tracks the dropout voltage as the load current varies, but also scale linearly with the load current. SHUTDOWN The SEMP8965 enters the sleep mode when the SHDN pin is low. When this occurs, the pass transistor, the error amplifier, and the biasing circuits, including the bandgap reference, are turned off, thus reducing the supply current to typically 1nA. Such a low supply current makes the SEMP8965 best suited for battery-powered applications. The maximum guaranteed voltage at the SHDN pin for the sleep mode to take effect is 0.4V. A minimum guaranteed voltage of 1.2V at the SHDN pin will activate the SEMP8965. Direct connection of the SHDN pin to the VIN to keep the regulator on is allowed for the SEMP8965. In this case, the SHDN pin must not exceed the supply voltage VIN. FAST START-UP Fast start-up time is important for overall system efficiency improvement. The SEMP8965 assures fast start-up speed when using the optional noise bypass capacitor (CC). To shorten start-up time, the SEMP8965 internally supplies a 500µA current to charge up the capacitor until it reaches about 90% of its final value. |
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